Model 238-B Antenna Matchbox
WARNING: DO NOT USE IT'S BUILT-IN BALUN
The built-in Balun in the Model 238B (and Models 238 and 238A) is a 4:1 Ruthroff balun and totally worthless for this application. DO NOT USE IT.
Instead, wind your own 1:1 Guanella Balun, either using RG-142 (or similar) Teflon-insulated coax, or bifilar winding. Use FT-240-43 core(s): 1 Core for 500w, 2 Cores for 1kW, 3 Cores for 1500W.